This invention relates to gradient coils in nuclear magnetic resonance imaging systems.
Nuclear magnetic resonance (NMR) systems may be used as medical diagnostic tools. Generally, they consist of a region that holds the patient or sample surrounded by a set of coils. The outermost coil is often a superconducting DC coil which provides a strong, constant magnetic field that polarizes the nuclei in the molecules of the sample or the patient's body. A smaller RF coil sits concentrically within the DC coil, and generates a time-varying RF field that excites the nuclei and detects their response to this excitation. A coil assembly, generally referred to as the "gradient coil", occupies the space between the above coils and generates a time-varying spatially dependent audio frequency magnetic field (usually a linear gradient field) which causes the response frequency of the nuclei to depend on their positions within the field. This imparted spatial information allows the generation of an image of the areas excited and detected by the RF coil.
Such a gradient coil assembly usually consists of a set of three primary coils (X, Y and Z) which may each be manufactured by winding a pattern of conductors on the surface of a cylinder. Each of these coils may be shielded with another pattern of conductors wound on another cylindrical surface which surrounds the coils. Coils of this type are presented in U.S. Pat. Nos. 4,737,716 and 4,794,338 (incorporated by reference), respectively entitled "Self-Shielded Gradient Coils for Nuclear Magnetic Resonance Imaging" and "Balanced Self-Shielded Gradient Coils", both of Roemer et al.
Gradient coils, because they operate at audio frequencies, can be the source of high levels of acoustic noise. This is particularly true for coils used in high speed imaging, as higher frequencies and higher currents are used.
It is also known to use Litz wire in transformers and coils as it reduces skin effect conduction and thus exhibits decreased resistance for a given conductor diameter. U.S. Pat. No. 4,549,042 entitled "Litz Wire for Decreasing Skin Effect at High Frequency" of Akiba discusses this effect.